As double-stranded DNA is heated a temperature is reached at that the two reaction strands divided.  This procedure is called as denaturation.  The temperature at that half of the DNA molecules have denatured is called as the melting temperature or Tm for that DNA if the temperature  is now lowered  and falls below the Tm, the two corresponding  strands  will form hydrogen  bonds with each  other  once  more  to  reform  a  double-stranded   molecule.  This procedure is called as renaturation   (or reannealing).   In reality, double-stranded   structures can form between any two single-stranded nucleic acid molecules (DNA-RNA, DNA-DNA, RNA-RNA) provided in which they have sufficient complementary nucleotide series to make the double-stranded molecule stable under the conditions used.  The general name given to these procedures is hybridization and the double-stranded nucleic acid product is called as a hybrid.